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Solvent effects on the solvatochromism of
7-aminocoumarins in neat and binary solvent
mixtures Correlation between the electronic
transition energies and the solvent polarity
parameters
Jin-Young Choi, T. J. Kang Department of
Chemistry, Daegu University Gyoung san, 712-714
Korea
The change in the electronic absorption and
emission energies of 7-aminocoumarin derivatives
in binary solvent mixtures has been studied. The
electronic transition energy along with the
Stokes shift is correlated with the orientation
polarizability of the solvent as well as the
empirical solvent polarity parameters ET(30). It
is observed that the emission peak shift traces
polarity change very nicely in the binary solvent
mixtures. The emission transition is more
strongly depend on the solvent polarity than the
absorption. From the dependence of the
Sokesshift of 7-aminocoumarins with the solvent
polarity parameters and the ground state dipole
moment obtained by the semi-empirical AM1
calculations, the excited state dipole moment was
estimated. The fluorescence lifetime change of
7-aminocoumarins in binary solvent mixtures was
measured and the results are explained in terms
of molecular conformation and solvent polarity.
The study indicates the empirical solvent
polarity ET(30) is a good measure of microscopic
solvent polarity and it probes in general the
non-specific solvent interactions.
Structure of 7-aminocoumarin dyes
Solvent Polarity Parameters
Solvatochromism and the estimation of the dipole
moment
Solvation is important for understanding the
solvent effects on chemical and biochemical
processes. Biological systems in which various
physiological processes such as transportation,
signaling, metabolism are controlled by
solvation. Changes in local polarity by
solute-solvent interactions in biological systems
are related to malfunction or disease. A
variety of solvent polarity parameters proposed.
But, there is no such thing as a single polarity
parameter in explaining a multitude of solvent
effects.
? ?f(e,n) Orientation polarizability
? Lippert, Mataga
Effect of general solvent polarity
? Bakhshiev, Kawski

? Ravi et al
Correlation between ETN and ?f(e,n)
Correlation of Stokesshift with ?f(e,n) and ETN
ETN was measured for 14 aprotic solvents and 5
protic solvents, and plotted versus ?f(e,n). It
is noted that there is a linear correlation
between ETN and ?f(e,n) for aprotic solvents,
but large deviation is observed for protic
solvents.
Solvatochromic shift of 7-aminocoumarin dye
Estimation of the excited dipole moment
(a) Absorption and (b) fluorescence Spectra of
coumarin 102 were measured in dioxane/ water
binary solvent system.
The shape of the cavity in 7-amonocoumarin
dyes is not spherical but is very much
elliptical. Thus whenever the cavity radius is
required for calculations, it should be taken
very carefully.
They show batochromic shift as solvent polarity
increases.
Comparison of emission peak shifts with the
change of ETN
Compounds m1 (cm-1) m2 (cm-1) m0 (cm-1) a (Cavity Radius, Å) ?µ (D) µg (D) µe/µg µe (D)
f(e,n) C102 1656 4802 6.38 2.05 13.10
f(e,n) C481 1924 5169 6.24 2.19 13.64
ETN C102 2249 3.98 2.06 8.44
ETN C481 2552 4.00 2.22 8.46
Calculated from the molecular volume assuming
a spherical shape Semi-empirical
AM1 Calculation
Solvent polarity and fluorescence lifetime change

Plotting fluorescence lifetime against solvent
polarity indicates characteristic curvilinear
change for different coumarin fluorophores.

1. The solvent effect on the solvatochromism is
   correlated to the empirical polarity parameter
   ETN better than the bulk solvent parameter
   function. The ETN value measures microscopic
   solvent polarity and it seems to probe more or
   less non-specific solvent interactions.
2. The dipole moment 7-aminocoumarins increases by
   almost twice upon excitation. The cavity shape is
   assumed to be very much elliptical and the cavity
   radius is predicted to be much larger than the
   generally considered value for these
   7-aminocoumarins.
3. Preferential solvation is taking place in binary
   solvent mixture since the change of peak shift
   seems to reflect the change of ETN very closely.
4. The lifetime of coumarin 102 gradually increases
   with increasing solvent polarity, but the
   fluorescence lifetime of coumarin 481 rapidly
   decreases as the solvent polarity increases. This
   is attributed to the nonradiative decay process
   which involves the formation of twisted
   intramolecular charge transfer state.

Fig(a)(c), The empirical solvent polarity
parameter plotted as a function of X , the mole
fraction of the more polar component of binary
solvent mixtures.(d)-(f)the wavenumbers of the
emission peaks for coumarin 102 dissolved in
three binary solvent mixtures plotted as a
function of mole fraction of more polar solvent.
The microscopic solvent polarity is probed very
nicely by an empirical polarity measure of ETN.
(a) and (d) benzene/acetonitrile mixture, (b)
and (e) benzene/methanol mixture, (c) and (f)
dioxane/water